In recent years, semiconductor devices have been highly integrated while achieving an increased operating speed. The degree of integration has been extremely improved particularly in LSI. Thus, the heat radiation property of the material of the substrate for carrying a semiconductor element has been increasingly emphasized.
A ceramic material for such an IC substrate has been generally made of alumina (Al.sub.2 O.sub.3). However, a conventional alumina sintered body has a low thermal conductivity and an insufficient heat radiation property. Thus, it is difficult to use such an alumina sintered body as a ceramic material for making a substrate, which can sufficiently cope with an increase in the calorific power of an IC chip.
In place of such an alumina substrate, attention has recently been directed to a substrate or a heat sink which is made of aluminum nitride having a high thermal conductivity, and much effort has been made to put the same into practice.
Aluminum nitride is a material essentially having a high thermal conductivity and a high insulation ability, with no toxicity dissimilarly to beryllia, and which does have a high thermal conductivity but is toxic. Thus, aluminum nitride is suitable as an insulating material or a package material for a semiconductor device. However, an aluminum nitride sintered body having the aforementioned characteristics is disadvantageous since it does not provide a satisfactory junction strength with respect to a metal or vitreous material. An aluminum nitride sintered body may be provided with a metallized layer on its surface, by a thick film forming method of directly applying commercially available metallized paste onto the surface of the aluminum nitride sintered body, or by a thin film forming method whereby a metal or an active metal layer is formed by a technique such as a vapor deposition. For practical purposes, so-formed metallized layers do not attain a sufficient junction strength with respect to the aluminum nitride sintered body. In practice, therefore, it is necessary to reform the surface of the aluminum nitride sintered body by some technique before or during the metallization step, in order to improve the junction strength between the metallized layer and a metal, for example.
In a well-known conventional method for such reforming, an oxide layer is formed by oxidizing a surface of an aluminum nitride sintered body.
For example, Japanese Patent Publication No. 58-11390 (1983) discloses a method of forming an oxide layer of SiO.sub.2, Al.sub.2 O.sub.3, mullite, Fe.sub.2 O.sub.3 and the like on a surface of an aluminum nitride sintered body. However, although such an oxide layer has an excellent affinity with respect to a glass layer, an alumina layer etc., it may be inferred that the same has a small affinity with respect to the aluminum nitride sintered body itself and has a questionable reliability. The term "reliability" in this context means that the junction or peel strength between the oxide layer and the aluminum nitride sintered body involves no "dispersion", in other words is uniform over the entire surface, so that a constant junction or peel strength can be maintained in a prescribed heat cycle test, and the like.
On the other hand, Japanese Laying-Open No. 63-115393 (1988) discloses a method of applying a conductive paste which is mainly composed of tungsten and/or molybdenum and an agent for reinforcing the junction strength prepared from an oxide mixture of SiO.sub.2, AlO.sub.2 and CaO onto an aluminum nitride sintered body and thereafter firing the same at a temperature of at least 1600.degree. C. However, the problem of this method is that the firing temperature is too high and the so-formed metallized layer has a rather insufficient reliability.